The present invention relates to a cathode ray tube, and, more particularly, to a shortened cathode ray tube having an envelope which has a sufficient mechanical strength in a narrowed deflection yoke mounting area.
In general, a cathode ray tube which is employed as an image display device is constituted by a vacuum envelope which is formed by connecting a panel portion which forms a screen having a phosphor coating on an inner surface thereof, a neck portion which accommodates an electron gun, and a funnel portion having a funnel shape which gradually decreases in diameter in the direction from the panel portion to the neck portion.
In a color cathode ray tube, a color screen formed of a plurality (usually three colors) of phosphors is provided on the inner surface of the panel portion, a shadow mask which operates as a color selection electrode is arranged adjacent to the screen, and an inline-type electron gun which irradiates three electron beams is accommodated in the neck portion.
The color cathode ray tube includes a stem at the end of the neck portion, which stem supports the accommodated electron gun and is provided with stem pins which supply a given voltage or given signals to the electron gun and are mounted in an annular manner, thus sealing the neck portion. A deflection yoke which scans the electron beams on the screen by deflecting the electron beams in both horizontal and vertical directions is mounted on the outer surface of the funnel portion.
A color display tube (CDT) used as a monitor device of an information processing terminal is used with a higher deflection frequency than a conventional cathode ray tube for television, and hence, the deflection power is increased.
In such a cathode ray tube, as a means for reducing the power consumed by the deflection yoke, the outer diameter size of a portion on which the deflection yoke of the funnel is mounted (deflection yoke mounting region) may be reduced in size so as to bring the deflection yoke closer to the electron beams, thus more efficiently applying the deflection magnetic field to the electron beams.
However, in case the outer diameter of the deflection yoke mounting region is simply reduced in size, a portion of the funnel portion connected to the neck portion (smaller diameter portion of the funnel portion) becomes narrow, and hence, at the time that the electron beams are deflected through the maximum deflection angle, the electron beams impinge on the inner surface of the funnel portion, thus giving rise to a region on the phosphor screen where the electron beams do not reach (non-scanned portion).
In view of this problem, Japanese Laid-Open Patent Publication 10-144238 discloses a cathode ray tube in which the outer wall in the deflection yoke mounting region of the funnel portion has a pyramidal shape so as to narrow the distance between the deflection yoke and the electron beams, while also avoiding the occurrence of a non-scanned portion.
Furthermore, in the cathode ray tube having a shortened length in the tube axis direction so as to provide an increased deflection angle, the enlargement angle from the neck portion to the funnel portion is increased. In such a cathode ray tube, a so-called inflection edge is formed, which inverts the curving direction of the inner wall surface toward the panel portion side of the deflection yoke mounting region.
FIG. 10 is a cross-sectional view showing a condition of a panel portion and a funnel portion of a conventional cathode ray tube cut along the short axis of the tube. Numeral 1 indicates the panel portion, numeral 2 indicates a neck portion, numeral 3 indicates the funnel portion, numeral 3A indicates a portion of a deflection yoke mounting region, and numeral 3i indicates an inner wall of the funnel portion 3. FIG. 10(a) shows an enlarged view of the boundary between the deflection yoke mounting region of the funnel portion 3 and a panel-portion side inner wall, that is, a portion W of the wall including the inflection edge P where the inner surface angle changes from the inner wall of the deflection yoke mounting region to the panel-portion side inner wall of the funnel portion.
In the conventional cathode ray tube, as shown in FIG. 10, where the tube axis is represented by the line Z-Z, a straight line in the short axis direction which passes through the inflection edge P and is perpendicular to the tube axis Z-Z is represented as y-y, and a tangent line which passes through a point which intersects the straight line y-y of the inflection edge P is represented as a, an angle xcex8 made by the straight line y-y and the tangent line a is set to a value not less than 11xc2x0.
Particularly, in a cathode ray tube having a contour in the deflection yoke mounting region in the form of a pyramidal shape, the vacuum envelope decreases its strength at a connection portion between the funnel portion and the neck portion, and hence, the possibility that a so-called implosion may occur is increased. In the above-mentioned Japanese Laid-Open Patent Publication 10-144238, to prevent the occurrence of an implosion, a reinforcing member is mounted on the connecting region between the pyramidal-shaped deflection yoke mounting region and the panel portion. In the cathode ray tube disclosed in Japanese Laid-Open Patent Publication 10-144238, the cross section of an outer wall thereof perpendicular to the tube axis of the deflection yoke mounting region is formed to have a rectangular shape and the cross section of an inner wall thereof is also formed to have a similar rectangular shape.
Further, in a cathode ray tube disclosed in Japanese Utility Model 44-29152, in case the deflection angle of the electron beams is large, to eliminate the above-mentioned non-scanned portion caused by the opening shape of a connecting portion between the funnel portion and the neck portion (narrow-diameter portion of the funnel portion) and to obviate implosion, the opening shape of the inner wall (cross section of the inner wall perpendicular to the tube axis) of the portion where the diameter of the funnel portion is narrowed is formed such that bulges which protrude inwardly are formed (in a so-called pin-cushion shape) on given portions of all of or two parallel sides out of four sides which form the profile line, and the corners are rounded.
In the conventional cathode ray tube, the angle xcex8 made by the short axis, the long axis or the diagonal axis (of the rectangular cross-section) which are perpendicular to the tube axis, or the above-mentioned straight line y-y of the inner surface of the funnel portion on the respective axes and the tangent line a of the inner surface of the funnel portion, typically exceeds 11xc2x0. Therefore, in a cathode ray tube in which the deflection yoke mounting region is formed to have a pyramidal shape, the shape of the inner wall surface of the funnel portion from the large diameter portion of the panel portion side to the front end of the neck portion (small diameter portion of the funnel portion) via the deflection yoke mounting region cannot have a gentle profile. Particularly, an inflection edge where the inner wall surface changes its angle is formed at the side end portion of the panel portion of the deflection yoke mounting region of the funnel portion.
In the case of a cathode ray tube whose deflection yoke mounting region adopts a pyramidal funnel shape, it is necessary to make the angle (xcex8) of the tangent line at the inflection edge small to assure the mechanical strength of the funnel portion. In case the above-mentioned angle is made smaller, in an internal graphite film coating process, which constitutes one of the cathode ray tube""s manufacturing steps, a coating liquid of graphite in the form of a film is settled on the inflection edge or it becomes difficult to smoothly coat the inner surface of the deflection yoke mounting region and the neck portion, and hence, the thickness of the internal graphite film becomes non-uniform due to the unevenness of the coating and the formation of a liquid well, so that a manual correction operation becomes necessary. Here, the internal graphite film has a function of supplying to the anode electrode of the electron gun a high voltage from an anode button which passes through the wall surface of the large diameter portion of the funnel from the front surface to the back surface thereof.
This internal graphite film is formed by a coating liquid in which graphite particles are dispersed in a solvent over the inner surface of the funnel portion and drying the coated liquid before connecting the panel portion to the funnel portion. That is, the funnel is rotated in a condition in which the funnel is vertically disposed with the large diameter side to which the panel portion is to be connected directed upwardly and the neck portion directed downwardly, and the internal graphite film is coated on the inner surface by means of an automatic brush coating machine.
Further, in the cathode ray tube which has a reduced length in the tube axis direction resulting in an increased deflection angle, the inner wall surface of the funnel portion on the diagonal line of the panel portion is remote from the tube axis, and hence, it is difficult to coat the graphite solution on this internal portion or the graphite before drying is concentrated on the corner portions so that the film thickness becomes uneven, thus giving rise to peeling off of the graphite film, or the graphite before drying sags in the inside of the neck portion, and hence, it becomes difficult to coat the internal graphite to a desired thickness. No consideration has been given for solving these problems with respect to conventional cathode ray tubes.
A color cathode ray tube of the present invention includes a vacuum envelope comprised of a panel portion having an approximately rectangular shape on which a phosphor film is formed on an inner surface thereof, a neck portion having a circular cross section which accommodates an electron gun, and a funnel portion having an opening cross section which is gradually enlarged toward a panel portion side thereof and is gradually reduced toward a neck portion side and includes a deflection yoke mounting region having a pyramidal shape at a side which is connected to the neck portion. An inflection edge at which the inner surface angle changes from an inner wall of the deflection yoke mounting region to a panel portion side inner wall of the funnel portion is formed on an inner surface of the funnel portion at the panel portion side terminal end of the deflection yoke mounting region, and an angle made by a straight line which passes through the endmost face of the inflection edge and is perpendicular to the tube axis and a tangent line at the endmost face is within 5xc2x0-11xc2x0.
In the pyramidal deflection yoke mounting region of the color cathode ray tube of the present invention, the inner wall opening thereof which is perpendicular to the tube axis direction is formed in a pin-cushion shape. Recesses having a curvature are formed in respective corners. The distance between a first straight line which connects bottom portions of neighboring recesses of the corner portions and a second straight line which is in contact with a central point of an inner wall between the neighboring corners and is parallel with the first straight line is not more than 2.0 mm and preferably not more than 1.0 mm, provided that the direction toward the tube axis is taken as +, and the relationship between a radius of curvature R1 of the cross section of an outer wall at a reference line of a deflection yoke mounting region and a radius of curvature R2 of the cross section of the inner wall is set such that R1 xe2x89xa7100 mm and R2xe2x89xa7R1. Further, in the color cathode ray tube of the present invention, the outer diameter size of the neck is set to not less than 22.5 mm and less than 29.1 mm, and the distance between the center beam and the side beam of the electron gun is set to not more than 5.0 mm, and the deflection angle of the electron beam is not less than 90xc2x0.
Further, in the color cathode ray tube of the present invention, an angle made by a straight line which passes through an endmost face of the inflection edge and is perpendicular to the tube axis and a tangent line at the endmost face is 5xc2x0-11xc2x0. The color cathode ray tube includes a pyramidal deflection yoke mounting region, and at a position of the reference line set in the deflection yoke mounting region, the cross section of the outer wall which is perpendicular to the tube axis has an approximately rectangular shape, and the opening of the inner wall which is perpendicular to the tube axis has an approximately barrel shape. The radius of curvature of the cross section of the outer wall is set as R1 (mm) and the radius of curvature of the cross section of the inner wall is set as R2 (mm), with R1xe2x89xa7100 mm and R2xe2x89xa7R1, and the outer diameter size of the neck is set to not less than 22.5 mm and less than 29.1 mm. The distance between the center beam and the side beam of the electron gun is set to not more than 5.0 mm, and the deflection angle of the electron beam is not less than 90xc2x0.
Due to the respective constitutions set forth above, the present invention can provide a color cathode ray tube which can assure a sufficient mechanical strength in the vacuum envelope, which is provided with a pyramidal deflection yoke mounting region at the funnel portion, and simultaneously can facilitate a reliable internal graphite film coating operation, can reduce the power consumption, and can accommodate a shortened tube axial length. According to the present invention, the coating of the internal graphite film formed on the inner wall of the funnel portion is facilitated, and hence, an internal graphite film having a uniform thickness and which is free from peeling-off can be formed, thus providing a highly reliable cathode ray tube.
With a view toward enhancing the mechanical strength of the cathode ray tube, it is advantageous to make the angle xcex8 at the inner surface of the funnel portion small. This is because, with a smaller angle xcex8, the inflection edge formed between the funnel portion and the transition portion of the deflection yoke mounting region of the funnel portion forms a step or a level difference, and hence, the vibration of an impact caused at the panel portion of the cathode ray tube will hardly be transmitted to the neck portion through the funnel portion, or the cross-sectional area at the step can be made as small as possible.
According to the present invention, by setting the angle xcex8 to not more than 11xc2x0, which is smaller than that of the conventional cathode ray tube, the mechanical strength can be enhanced. To demonstrate is, the inventors have prepared cathode ray tubes which are set to various angles xcex8 as samples and have found that in case the angle xcex8 is set to not more than 11xc2x0, the required mechanical strength could be obtained.
On the other hand, the inventors have found that in case the angle xcex8 is set to less than 5xc2x0, in the internal graphite film coating operation, when the tube axis of the cathode ray tube is held in a vertical direction, the funnel portion assumes substantially a horizontal condition, and hence, a coating liquid well is liable to occur. In case a graphite coating liquid well is present in the panel, the thickness of the internal graphite film becomes uneven, and hence, the emission characteristics of the cathode ray tube are deteriorated, or the film is peeled off and peeled-off pieces become foreign matter in the tube, which gives rise to poor withstand voltage characteristics.
The present invention is not limited to the above-mentioned constitutions an the constitutions of embodiments which will be explained hereinafter, and so various modifications can be considered without departing from the technical spirit of the present invention.